1. Field of the Invention
The present invention is related to semiconductor integrated circuits, and specifically, to an active low pass filter having a tuning circuit to minimize the impact of temperature and process variations.
2. Background Art
Today mixed-signal integrated circuits comprise both analog and digital components on a single chip. Such mixed-signal integrated circuits include devices such as transistors, capacitors, resistors, inductors, and the like. These devices are coupled together in a plethora of ways to form simple components, such as logic gates, registers and memory cells, as well as more complicated components, including entire microprocessors, memory arrays, amplifiers, and the like.
The frequency response of active filters is determined by the values of their various resistance-capacitance (RC) elements. Although switches, small-value capacitors, and operational amplifiers can be realized in integrated circuits using MOS technology, it is very difficult to make resistors and capacitors with the values and accuracy required by certain radio frequency (RF) applications.
This integration drawback has been overcome by implementing resistors with MOS capacitors coupled between MOS switches that are rapidly switched on and off. Such devices are called xe2x80x9cswitched capacitors.xe2x80x9d Switched-capacitors can commonly be used in electronic applications to realize a wide variety of active filters that have the advantages of compactness and tunability. They are typically used to replace resistors in active filter circuits. The time constants arising from these simulated resistances and other MOS capacitors are based on capacitance ratios. Providing values set by capacitor ratios to control the time constants yields on-chip RC components that are less susceptible to errors due to manufacturing process variations, and temperature changes. However, the usefulness of switched-capacitors with operational amplifiers circuits is limited. The DC offset voltages plagued by non-ideal operational amplifiers causes loss of accuracy at low operating voltage levels.
What is needed is a switched-capacitor circuit technique for eliminating the adverse impact of operational amplifier DC offset voltages.
The present invention is directed to an integrated circuit formed on a semiconductor chip comprising a low pass filter circuit and a tuning circuit. The low pass filter circuit has a first resistor of a first resistance value and a capacitor of a first capacitance value, wherein the first resistance value and the first capacitance value determine a corner frequency of the filter.
The tuning circuit has a second resistor of a second resistance value, a switched-capacitor of a third resistance value and a comparator that compares two voltage signals and produces a control signal, wherein the control signal adjusts the first and second resistance values as a function of the third resistance value.
In one embodiment of the integrated circuit, a first one of the two voltage signals is coupled to the switched-capacitor and a second one of the two voltage signals is coupled to the comparator, wherein the integrated circuit further comprises means for adjusting the corner frequency of the filter by varying at least one of the two voltage signals.
In another embodiment of the integrated circuit, the integrated circuit includes a clock to control the switched-capacitor, wherein the corner frequency of the filter can be adjusted by varying frequency of the clock.
In yet another embodiment, the corner frequency of the filter can be changed by adjusting the corner frequency and by adjusting at least one of the two voltage signals.
These and other advantages and features will become readily apparent in view of the following detailed description of the invention.